1. Field of the Invention
This invention relates to spring suspended ring laser gyroscopes and, more particularly, to an improved method of producing dither in such a gyroscope through the use of feedback.
2. Description of the Prior Art
A variety of types of ring laser gyroscopes have been developed. Typical is the apparatus disclosed in U.S. Pat. No. 3,373,650 where a ring laser gyroscope is shown which employs monochromatic beams of light traveling in two opposite directions around a closed loop path about the axis of rotation. Rotation of the apparatus about the axis of rotation causes the effective path length for each beam to change and results in oscillation at different frequencies in the beams since the frequency of oscillation of a laser depends upon the length of a lasing path. The two waves may be combined to generate interference patterns from which a measure of the rotational rate about the axis can be obtained. As was explained in this patent, the difference in frequency between the two beams at low rotational rates is small and they tend to resonate together, or to "lock-in", and oscillate at only one frequency. Therefore, low rotation rates cannot be detected. In U.S. Pat. No. 3,373,650, this problem is overcome by oscillating or "dithering" the apparatus to avoid lock-in of the two beams. Another structure of this kind is disclosed in U.S. Pat. No. 3,467,472 and a detailed explanation of the problem and the various solutions proposed thereto is contained in U.S. Pat. No. 3,879,130. The latter patent takes a different approach to the problem and describes the use of a saturable absorber placed in the ring laser cavity as a solution to the problem. The dither systems described above are mechanical in nature and in them operation has been "open loop". An improved system, employing feedback, is described in U.S. Pat. No. 4,132,482. This system, while successful to a large degree in reducing the amount of residual lock-in remaining in the system and producing less error than the open loop dithering systems described above, has the disadvantage that it cannot be used with a spring suspension system such as that shown in U.S. Pat. No. 3,373,650. Such systems usually are high Q in order to maintain the amplitude of the dither without using much energy. When feedback is used, the torquer would have to supply an inordinately large amount of power in order to force a change in the dither frequency established by the torsion spring and the inertia of the gyroscope.